Currently, medical probes, such as catheters and surgical probes, are used to treat heart abnormalities, such as atrial fibrillation and other cardiac arrythmias. In a typical procedure, a medical probe employs one or more ablation elements and one or more corresponding temperature sensors in order to therapeutically ablate tissue within the heart in a controlled manner. Temperature sensors currently used in medical probes, such as thermistors or thermocouples, all require separate analog signal conditioning circuitry for each sensor, although analog to digital (A/D) conversion circuitry may be multiplexed.
Thermistors respond to changes in temperature with a well-defined change in resistance. Analog conditioning circuitry, which is used to interface the thermistors with the A/D converter, measures the resistance of the thermistors, and thus, the temperature of the adjacent tissue, by separately measuring the voltage across each thermistor in response to a constant current. By comparison, thermocouples contain a junction of dissimilar metals that generate a small voltage proportional to temperature, due to the Peltier effect. Analog conditioning circuitry is connected to each thermocouple to amplify the voltage output thereby and to reduce any noise associated with such voltage.
Further, to support such multiple sensor probes, current technology requires a large number of wires to be contained within the small and limited space of the probe body, thereby rendering manufacture of such medical probes increasingly difficult. This constraint is even more pronounced in catheters, the diameters of which must be minimized to allow the catheters to be introduced into the heart through the vasculature of a patient. The increased number of wires in connectors and cabling also makes the manufacture of accessory cables used to support multiple sensor medical probes more difficult and expensive. Moreover, connector reliability is reduced due to the large number of connections required to implement discrete wires for each temperature sensor.
Regardless of the type of sensor utilized, the analog conditioning circuitry must be duplicated with the currently available designs for each sensor. For example, FIG. 29 illustrates a prior art system, which includes a power generator 66 that is coupled to a medical probe 50 via a cable 55. A standard generator interface 62 is used to interface the proximal end of the cable 55 to the circuitry within the generator 66, and a standard probe interface 62 is used to interface the distal end of the cable 55 to the circuitry within the medical probe 50. The power generator 66 includes a power source 51 (in this case an RF oscillator), which provides RF power to ablation energy electrodes 53 located at the distal end of the medical probe 50. The power generator 66 further includes a temperature controller 54 (in this case, a microprocessor), which communicates with analog temperature sensors 52 located at the distal end of the medical probe 50 via parallel sets of analog to digital converters 56 and signal conditioners 60. As illustrated, a separate analog to digital converter 56 and signal conditioners 60 is required for each temperature sensor 52.
FIG. 30 illustrates another prior art system, which includes a power generator 68 that is coupled to the medical probe 50 via the cable 55. The power generator 68 differs from the power generator 66 shown in FIG. 29 in that the power generator 68 uses a single analog to digital converter with multiplexing capability 58 to process signals from each sensor 52.
The additional circuitry required for each sensor 52 generally involves expensive, low noise integrated circuits. Time consuming calibration of each input during manufacturing is also typically required. As a result, the amount of circuit duplication increases by the number of sensors that must be read, thereby making systems with more than a few temperature sensors expensive and impractical. Also, the ablation power generators that support these medical probes are necessarily designed in a non-optimal manner. For a multiple sensor medical probe, the ablation power generators must be designed to accommodate the number of expected sensors by providing separate analog inputs for each sensor, as illustrated in FIGS. 29 and 30. Therefore, when designing such power generators, a tradeoff must be made between the excessive costs of providing extra sensor inputs to accommodate future requirements and the risk of premature obsolescence of a power generator that provides too few sensor inputs.
Moreover, the sensors are typically located from between ten to fifty feet away from the ablation power generators, being connected through fine-gauge wire in the medical probe itself, and through one or more cables with intermediate connections. The analog voltages which represent the temperature are typically quite small, particularly with thermocouples, where the dynamic range in the area of interest is usually only in the hundreds of microvolts. These analog voltages are susceptible to electrical noise induced by ablation power and sources of electromagnetic interference in the environment, some of which may be of a high enough amplitude or low enough frequency range that filtering may not be practical.
Consequently, there is a need to provide a medical probe system that contains a reduced number of electrical paths, or temperature sensor wires, as well as a medical probe system that outputs temperature sensor signals that exhibit little or no noise.